Polyolefins such as polyethylene and polypropylene may be prepared by particle form polymerization, also referred to as slurry polymerization. In this technique, feed materials such as monomer and catalyst are fed to a loop reactor, and a product slurry containing solid polyolefin particles in a liquid medium is taken off or withdrawn from the reactor.
In a loop polymerization operation, a fluid slurry is circulated around the loop reactor using one or more pumps, typically axial flow pumps having impellers disposed within the reactor. The pumps provide the motive force for circulation of the fluid slurry. As the volume of the reactor and the solids concentration of the fluid slurry increase, the demands on the pumps also increase. In general, the flow rate, pressure, density, and viscosity of the fluid slurry must be considered in selecting and operating the loop reactor pumps.
Slurry polymerization in a loop reaction zone has proven commercially successful. The slurry polymerization technique has enjoyed international success with billions of pounds of olefin polymers being so produced annually. However, it is still desirable to design and build larger reactors. The size of a reactor has a significant impact on the pump requirements, particularly as to the head (differential pressure across the impeller of the pump, expressed in feet of liquid) and flow (velocity multiplied by cross-sectional area of the pipe, expressed in gallons per minute or GPM) developed by the pump.
Until fairly recently, fluid slurries of olefin polymers in a diluent were generally limited to relatively low reactor solids concentrations. Settling legs were used to concentrate the slurry to be withdrawn, so that at the exit of the settling legs, the slurry would have a higher solids concentration. As the name implies, settling occurs in the setting legs to increase the solids concentration of the slurry to be withdrawn.
In addition to the concentration of the slurry, another factor affecting the solids concentration in the reactor is fluid slurry circulation velocity. A higher slurry velocity for a given reactor diameter allows for higher solids, since the slurry velocity affects such limiting factors as heat transfer and reactor fouling due to polymer build up in the reactor.
By increasing the head and flow capability of the loop reactor circulating pump(s), one can circulate a higher weight percent solids in the reactor. The use of two pumps in series may allow a doubling of pumping head capability and a resulting solids increase. The two pumps may be located on different segments of the reactor and it may be desirable for each pump to be dedicated to an even number of legs.